Ruby L. C. Hoo

Angiopoietin-like protein 4 decreases blood glucose and improves glucose tolerance but induces hyperlipidemia and hepatic steatosis in mice

Angiopoietin-like protein 4 (ANGPTL4) is a circulating protein predominantly expressed in adipose tissue and liver. Several recent studies demonstrated that ANGPTL4 is the target gene of peroxisome proliferation activators, the agonists of which are widely used as the antidiabetic and lipid-lowering drugs. Here we provide evidence that ANGPTL4 is a blood-borne hormone directly involved in regulating glucose homeostasis, lipid metabolism, and insulin sensitivity. Adenovirus-mediated expression of ANGPTL4 potently decreased blood glucose and improved glucose tolerance, whereas it induced hyperlipidemia, fatty liver, and hepatomegaly in C57 mice. In db/db diabetic mice, ANGPTL4 treatment reduced hyperglycemia to a normal level, and markedly alleviated glucose intolerance and hyperinsulinemia. Ex vivo studies on primary rat hepatocytes revealed that ANGPTL4 significantly decreased hepatic glucose production and enhanced insulin-mediated inhibition of gluconeogenesis. Serum levels of ANGPTL4 in human subjects inversely correlated with plasma glucose concentrations and HOMA IR, the homeostasis model assessment of insulin resistance. In patients with type 2 diabetes, serum levels of ANGPTL4 were significantly lower than those in healthy subjects, suggesting that the decreased ANGPTL4 could be a causative factor of this disease. These results collectively indicate that ANGPTL4 exerts distinct effects on glucose and lipid metabolism, and that its beneficial effect on glucose homeostasis might be useful for the treatment of diabetes.

Adiponectin Modulates the Glycogen Synthase Kinase-3β/β-Catenin Signaling Pathway and Attenuates Mammary Tumorigenesis of MDA-MB-231 Cells in Nude Mice

Adiponectin is an adipokine that has pleiotropic beneficial roles in systemic insulin resistance and inflammation. Several recent clinical studies suggest that low serum levels of adiponectin are associated with increased risks of breast cancer. Here, we investigated the direct effects of adiponectin on breast cancer development in vitro and in vivo. Our results showed that adiponectin significantly attenuated the proliferations of two typical human breast cancer cells, MDA-MB-231 and T47D, in a cell type-specific manner. Further analysis revealed that adiponectin could induce apoptosis and arrest the cell cycle progression at G(0)-G(1) phase in MDA-MB-231 cells. Prolonged treatment with adiponectin in this cell line blocked serum-induced phosphorylation of Akt and glycogen synthase kinase-3beta (GSK-3beta), suppressed intracellular accumulation of beta-catenin and its nuclear activities, and consequently reduced expression of cyclin D1. Adiponectin-mediated suppression of cyclin D1 expression and attenuation of cell proliferation was abrogated by the GSK-3beta inhibitor lithium chloride. These results suggest that the inhibitory role of adiponectin on MDA-MB-231 cell growth might be attributed to its suppressive effects on the GSK-3beta/beta-catenin signaling pathway. Furthermore, our in vivo study showed that both supplementation of recombinant adiponectin and adenovirus-mediated overexpression of this adipokine substantially reduced the mammary tumorigenesis of MDA-MB-231 cells in female nude mice. Taken together, these data support the role of adiponectin as a negative regulator of breast cancer development and also suggest that adiponectin might represent a novel therapeutic target for this disease.

Physical exercise-induced hippocampal neurogenesis and antidepressant effects are mediated by the adipocyte hormone adiponectin.

Significance This study unmasks a previously unidentified functional role of adiponectin (a hormone secreted by adipocytes) in modulating hippocampal neurogenesis and alleviating depression-like behaviors. To our knowledge, this is the first report showing that adiponectin may be an essential factor that mediates the antidepressant effects of physical exercise on the brain by adiponectin receptor 1-mediated activation of AMP-activated protein kinase. Our results reveal a possible mechanism by which exercise increases hippocampal neurogenesis and also suggest a promising therapeutic treatment for depression. Adiponectin (ADN) is an adipocyte-secreted protein with insulin-sensitizing, antidiabetic, antiinflammatory, and antiatherogenic properties. Evidence is also accumulating that ADN has neuroprotective activities, yet the underlying mechanism remains elusive. Here we show that ADN could pass through the blood–brain barrier, and elevating its levels in the brain increased cell proliferation and decreased depression-like behaviors. ADN deficiency did not reduce the basal hippocampal neurogenesis or neuronal differentiation but diminished the effectiveness of exercise in increasing hippocampal neurogenesis. Furthermore, exercise-induced reduction in depression-like behaviors was abrogated in ADN-deficient mice, and this impairment in ADN-deficient mice was accompanied by defective running-induced phosphorylation of AMP-activated protein kinase (AMPK) in the hippocampal tissue. In vitro analyses indicated that ADN itself could increase cell proliferation of both hippocampal progenitor cells and Neuro2a neuroblastoma cells. The neurogenic effects of ADN were mediated by the ADN receptor 1 (ADNR1), because siRNA targeting ADNR1, but not ADNR2, inhibited the capacity of ADN to enhance cell proliferation. These data suggest that adiponectin may play a significant role in mediating the effects of exercise on hippocampal neurogenesis and depression, possibly by activation of the ADNR1/AMPK signaling pathways, and also raise the possibility that adiponectin and its agonists may represent a promising therapeutic treatment for depression.

Growth Hormone Induces Hepatic Production of Fibroblast Growth Factor 21 through a Mechanism Dependent on Lipolysis in Adipocytes

Background: Both growth hormone (GH) and FGF21 are fasting-inducible hormones involved in modulation of lipolysis. Results: GH injection caused a rapid elevation of circulating FGF21 in mice, and such an effect was blocked by the lipolysis inhibitor niacin. FGF21 knock-out mice exhibited greater potency in GH-induced lipolysis. Conclusion: GH and FGF21 form a negative feedback loop to fine-tune lipolysis in adipocytes. Significance: This study provides a novel insight on hormonal control of lipolysis. Fibroblast growth factor (FGF) 21 and growth hormone (GH) are metabolic hormones that play important roles in regulating glucose and lipid metabolism. Both hormones are induced in response to fasting and exert their actions on adipocytes to regulate lipolysis. However, the molecular interaction between these two hormones remains unclear. Here we demonstrate the existence of a feedback loop between GH and FGF21 on the regulation of lipolysis in adipocytes. A single bolus injection of GH into C57 mice acutely increases both mRNA and protein expression of FGF21 in the liver, thereby leading to a marked elevation of serum FGF21 concentrations. Such a stimulatory effect of GH on hepatic FGF21 production is abrogated by pretreatment of mice with the lipolysis inhibitor niacin. Direct incubation of either liver explants or human HepG2 hepatocytes with GH has no effect on FGF21 expression. On the other hand, FGF21 production in HepG2 cells is significantly induced by incubation with the conditioned medium harvested from GH-treated adipose tissue explants, which contains high concentrations of free fatty acids (FFA). Further analysis shows that FFA released by GH-induced lipolysis stimulates hepatic FGF21 expression by activation of the transcription factor PPARα. In FGF21-null mice, both the magnitude and duration of GH-induced lipolysis are significantly higher than those in their wild type littermates. Taken together, these findings suggest that GH-induced hepatic FGF21 production is mediated by FFA released from adipose tissues, and elevated FGF21 in turn acts as a negative feedback signal to terminate GH-stimulated lipolysis in adipocytes.

Adiponectin prevents diabetic premature senescence of endothelial progenitor cells and promotes endothelial repair by suppressing the p38 MAP kinase/p16INK4A signaling pathway.

OBJECTIVE A reduced number of circulating endothelial progenitor cells (EPCs) are casually associated with the cardiovascular complication of diabetes. Adiponectin exerts multiple protective effects against cardiovascular disease, independent of its insulin-sensitizing activity. The objective of this study was to investigate whether adiponectin plays a role in modulating the bioavailability of circulating EPCs and endothelial repair. RESEARCH DESIGN AND METHODS Adiponectin knockout mice were crossed with db+/− mice to produce db/db diabetic mice without adiponectin. Circulating number of EPCs were analyzed by flow cytometry. Reendothelialization was evaluated by staining with Evans blue after wire-induced carotid injury. RESULTS In adiponectin knockout mice, the number of circulating EPCs decreased in an age-dependent manner compared with the wild-type controls, and this difference was reversed by the chronic infusion of recombinant adiponectin. In db/db diabetic mice, the lack of adiponectin aggravated the hyperglycemia-induced decrease in circulating EPCs and also diminished the stimulatory effects of the PPARγ agonist rosiglitazone on EPC production and reendothelialization. In EPCs isolated from both human peripheral blood and mouse bone marrow, treatment with adiponectin prevented high glucose–induced premature senescence. At the molecular level, adiponectin decreased high glucose–induced accumulation of intracellular reactive oxygen species and consequently suppressed activation of p38 MAP kinase (MAPK) and expression of the senescence marker p16INK4A. CONCLUSIONS Adiponectin prevents EPC senescence by inhibiting the ROS/p38 MAPK/p16INK4A signaling cascade. The protective effects of adiponectin against diabetes vascular complications are attributed in part to its ability to counteract hyperglycemia-mediated decrease in the number of circulating EPCs.

Mitochondrial dysfunction contributes to the increased vulnerabilities of adiponectin knockout mice to liver injury.

Adiponectin is an adipocyte‐derived hormone with a wide range of beneficial effects on obesity‐related medical complications. Numerous epidemiological investigations in diverse ethnic groups have identified a lower adiponectin level as an independent risk factor for nonalcoholic fatty liver diseases and liver dysfunctions. Animal studies have demonstrated that replenishment of adiponectin protects against various forms of hepatic injuries, suggesting it to be a potential drug candidate for the treatment of liver diseases. This study was designed to investigate the cellular and molecular mechanisms underlying the hepatoprotective effects of adiponectin. Our results demonstrated that in adiponectin knockout (ADN‐KO) mice, there was a preexisting condition of hepatic steatosis and mitochondrial dysfunction that might contribute to the increased vulnerabilities of these mice to secondary liver injuries induced by obesity and other conditions. Adenovirus‐mediated replenishment of adiponectin depleted lipid accumulation, restored the oxidative activities of mitochondrial respiratory chain (MRC) complexes, and prevented the accumulation of lipid peroxidation products in ADN‐KO mice but had no obvious effects on mitochondrial biogenesis. The gene and protein levels of uncoupling protein 2 (UCP2), a mitochondrial membrane transporter, were decreased in ADN‐KO mice and could be significantly up‐regulated by adiponectin treatment. Moreover, the effects of adiponectin on mitochondrial activities and on protection against endotoxin‐induced liver injuries were significantly attenuated in UCP2 knockout mice. Conclusion: These results suggest that the hepatoprotective properties of adiponectin are mediated at least in part by an enhancement of the activities of MRC complexes through a mechanism involving UCP2. (HEPATOLOGY 2008.)

Adiponectin Is Required for PPARγ-Mediated Improvement of Endothelial Function in Diabetic Mice

Rosiglitazone is a PPARγ agonist commonly used to treat diabetes. In addition to improving insulin sensitivity, rosiglitazone restores normal vascular function by a mechanism that remains poorly understood. Here we show that adiponectin is required to mediate the PPARγ effect on vascular endothelium of diabetic mice. In db/db and diet-induced obese mice, PPARγ activation by rosiglitazone restores endothelium-dependent relaxation of aortae, whereas diabetic mice lacking adiponectin or treated with an anti-adiponectin antibody do not respond. Rosiglitazone stimulates adiponectin release from fat explants, and subcutaneous fat transplantation from rosiglitazone-treated mice recapitulates vasodilatation in untreated db/db recipients. Mechanistically, adiponectin activates AMPK/eNOS and cAMP/PKA signaling pathways in aortae, which increase NO bioavailability and reduce oxidative stress. Taken together, these results demonstrate that adipocyte-derived adiponectin is required for PPARγ-mediated improvement of endothelial function in diabetes. Thus, the adipose tissue represents a promising target for treating diabetic vasculopathy.

Selective Elevation of Adiponectin Production by the Natural Compounds Derived from a Medicinal Herb Alleviates Insulin Resistance and Glucose Intolerance in Obese Mice

Adiponectin is an adipocyte-derived insulin-sensitizing hormone with antidiabetic, antiinflammatory, and antiatherosclerotic properties. A decreased serum level of adiponectin in obesity has been identified as an independent risk factor for diabetes and cardiovascular complications, suggesting that pharmacological intervention aimed at elevating adiponectin production might hold promise for the treatment and/or prevention of these diseases. Here we report the identification of two structurally related natural compounds (astragaloside II and isoastragaloside I) from the medicinal herb Radix Astragali that possess such an activity. Astragaloside II and isoastragaloside I selectively increased adiponectin secretion in primary adipocytes without any obvious effects on a panel of other adipokines. Furthermore, an additive effect on induction of adiponectin production was observed between these two compounds and rosiglitazone, a thiazolidinedione class of insulin-sensitizing drugs. Chronic administration of astragaloside II and isoastragaloside I in both dietary and genetic obese mice significantly elevated serum levels of total adiponectin and selectively increased the composition of its high molecular weight oligomeric complex. These changes were associated with an alleviation of hyperglycemia, glucose intolerance, and insulin resistance. By contrast, the beneficial effects of these two compounds on insulin sensitivity and glucose metabolism were diminished in adiponectin knockout mice. In conclusion, our results suggest that pharmacological elevation of circulating adiponectin alone is sufficient to ameliorate insulin resistance and diabetes and support the use of adiponectin as a biomarker for future drug discovery. The two natural compounds might provide the lead as a novel class of therapeutics for obesity-related diseases.

APPL1 potentiates insulin secretion in pancreatic β cells by enhancing protein kinase Akt-dependent expression of SNARE proteins in mice

Insulin resistance and defective insulin secretion are the two major features of type 2 diabetes. The adapter protein APPL1 is an obligatory molecule in regulating peripheral insulin sensitivity, but its role in insulin secretion remains elusive. Here, we show that APPL1 expression in pancreatic β cells is markedly decreased in several mouse models of obesity and diabetes. APPL1 knockout mice exhibit glucose intolerance and impaired glucose-stimulated insulin secretion (GSIS), whereas transgenic expression of APPL1 prevents high-fat diet (HFD)-induced glucose intolerance partly by enhancing GSIS. In both pancreatic islets and rat β cells, APPL1 deficiency causes a marked reduction in expression of the exocytotic machinery SNARE proteins (syntaxin-1, synaptosomal-associated protein 25, and vesicle-associated membrane protein 2) and an obvious decrease in the number of exocytotic events. Such changes are accompanied by diminished insulin-stimulated Akt activation. Furthermore, the defective GSIS and reduced expression of SNARE proteins in APPL1-deficient β cells can be rescued by adenovirus-mediated expression of APPL1 or constitutively active Akt. These findings demonstrate that APPL1 couples insulin-stimulated Akt activation to GSIS by promoting the expression of the core exocytotic machinery involved in exocytosis and also suggest that reduced APPL1 expression in pancreatic islets may serve as a pathological link that couples insulin resistance to β-cell dysfunction in type 2 diabetes.

Pharmacological inhibition of adipocyte fatty acid binding protein alleviates both acute liver injury and non-alcoholic steatohepatitis in mice

BACKGROUND & AIMS Adipocyte fatty acid binding protein (A-FABP) is a key mediator of inflammatory response in macrophages. Increased hepatic expression and circulating levels of A-FABP have been observed in patients with non-alcoholic fatty liver disease (NAFLD). Here, we investigated the role of A-FABP in both lipopolysaccaride (LPS)-induced acute liver injury and high fat high cholesterol (HFHC) diet-induced NAFLD in mice. METHODS Mice with LPS-induced acute liver injury and HFHC diet-induced obesity were treated with the A-FABP inhibitor BMS309403. Liver tissues of the mice were analyzed by immunohistochemistry, Western blot or real-time PCR. RESULTS A-FABP expression in Kupffer cells was significantly elevated in mice with LPS-induced acute liver injury and HFHC diet-induced obesity, as compared to their healthy controls. Pretreatment of mice with BMS309403 led to a diminished LPS-induced elevation in serum levels of alanine transaminase and hepatic production of pro-inflammatory cytokines. Likewise, chronic treatment of HFHC diet-induced obese mice with BMS309403 ameliorated hepatic steatosis, macrophage infiltration, and cellular ballooning of hepatocytes. Such improvements in liver function and morphology were accompanied by significantly decreased activation of both c-Jun and NF-κB. Pretreatment with BMS309403 suppressed both LPS- and palmitate-induced pro-inflammatory responses in isolated rat Kupffer cells. Adenovirus-mediated ectopic expression of A-FABP alone was sufficient to induce liver injury and inflammation in mice. CONCLUSIONS These findings suggest that A-FABP is an important contributor to both LPS-induced acute liver injury and diet-induced NAFLD by potentiating inflammation in Kupffer cells. Pharmacological inhibition of A-FABP may represent a promising modality for obesity-related non-alcoholic steatohepatitis.